The rotary rockbit and in particular the three coned bit is widely utilized in earth boring through geological formations for the drilling of water, oil and gas wells. It is well known in the art that such rockbits operate under extremely heavy loads and severe conditions of corrosion, temperature and abrasion. At the same time, the economics of petroleum production demand of such bits that they have a longer useful lifetime and improved performance since as is well known, the labor cost of replacing a worn or broken bit far exceeds the cost of the bit per se. This has led to improved materials and/or metallurgical treatments by which such goals have been substantially enhanced. By virtue of such improvements, failures have tended to increasingly become isolated to the bearing system about which the cutter element rotates relative to its support arm.
It will be appreciated that the precise operating conditions vary with the type of geological formation and properties of the bit being utilized. Whether operated under heavy load at high penetration rate or moderate load at relatively high speeds, the rockbits are subjected to a highly corrosive environment and temperature extremes. Since drilling is typically conducted thousands of feet underground, extreme temperature elevations are encountered for which a drilling fluid is continuously circulated to cool and flush the bit and carry away cuttings. The fluid per se contains a high content of water with added chemicals to control water loss or to control viscosity and/or PH factor. Some of these chemicals may result in a corrosive drilling fluid such that the drill cuttings, materials encountered in the earth formations, barites added for fluid weight control and/or the chemical composition of the drilling fluid combine to create a highly corrosive and abrasive drilling environment.
In recognition of the foregoing, considerable effort has been expended toward enhancing the bearing life in such rockbits. By and large such efforts have included metallurgical treatments whereby the bearing surfaces are characterized by increased endurance against wear and failure. For example, in U.S. Pat. No. 3,984,158 to Sorensen et al there is disclosed of a sintered bearing matrix compressed into the shape of the desired bearing element. An anti-galling material is infiltrated into the matrix which is then hardened to provide a bearing element having a hard wear resistant surface and areas of anti-galling material.
U.S. Pat. No. 3,885,838 to Childers et al discloses use of bearing surfaces which are selectively carburized and uncarburized to enhance wear resistance and to alleviate cracking respectively at controlled locations.
U.S. Pat. No. 4,021,084 to Garner discloses a construction in which either the solid journal bearing or its cooperating bearing support surface is provided with a body of hardened carburized steel. A plurality of mutually spaced apart shallow pockets extend from the bearing surface and are filled with a wear resistant material even harder than the hardened carburized steel and having low ductility.
Other patents, such as U.S. Pat. No. 3,842,921 to Dill et al describe various known metallurgical techniques for carburizing and boronizing of the rockbit cutter surfaces in contrast to the bearing surfaces thereof. Patents such as U.S. Pat. Nos. 2,367,978 to Troy and 3,661,820 to Foreman et al are non-rockbit related and more general in their disclosure of coating compositions for masking portions of metal from the effects of a treating composition.
While each of the above have contributed toward advances in the art, it will be appreciated that each is relatively complex and costly to implement. Yet to the extent they have been commercially utilized they have undoubtedly been justified on the basis of their contribution to the overall economic saving in the drilling system of which they have been a part. Despite recognition therefore of the need to further improve rockbit construction in order to enhance the economics of earth drilling, it has not heretofore been known how to effectively obtain the combination of significant cost reduction and enhanced operating performance in the bearing surface construction of a rotary rockbit.